This invention relates to a NOx-reducing adsorption unit having an adsorption bed, in which the engine exhaust and the stream of regeneration gas, including hydrogen and carbon monoxide, both flow continuously, the adsorption bed and a gas inlet distributor having continuous relative rotation, portions of the adsorption bed being in fluid communication with engine exhaust inlet manifold for a first fraction of each cycle and then being in fluid communication with the regeneration gas for another fraction of each cycle, whereby successive portions of the filter first adsorb engine exhaust and then are regenerated, continuously.
The Environmental Protection Agency (EPA) has set, for 2007 and beyond, vehicle internal combustion engine emission requirements; one exemplary requirement for diesel engines, is NOx and non-methane hydrocarbons below 0.20 grams bhp-hr and 0.14 grams/bhp-hr, respectively. This contrasts with current standards of 4.0 grams/bhp-hr and 1.3 grams/bhp-hr, respectively. Thus, the catalytic converters must accomplish a significant reduction in NOx.
Apparatus that oxidizes engine fuel to provide a mix that enhances NOx reduction is disclosed in U.S. Pat. No. 5,412,946, in PCT published application WO 01/34950, and U.S. patent application Publication 2001/41153.
In commonly owned U.S. patent application Ser. No. 10/159,369, filed May 31, 2002, moisture and possibly oxygen, derived from the exhaust of a hydrocarbon-fueled, internal combustion engine are processed along with fuel from the engine""s fuel tank in a fuel processor, which may be a catalytic partial oxidation reformer, a non-catalytic (homogeneous) partial oxidation reformer, or an auto thermal reformer, to generate a stream of hydrogen and carbon monoxide which is used to regenerate NOx traps following the formation of nitrogen-containing compounds by reaction of the exhaust with adsorbent in the NOx traps.
In FIG. 1, an engine 9 has a conventional turbo compressor 10 feeding an air inlet line 11, a hydrocarbon fuel tank 12, and a fuel pump 13. The fuel may be diesel fuel, gasoline, natural gas, liquid petroleum gas, or propane. The fuel is fed by a first line 17 to the engine for combustion with the air, and is fed by a second line 18 through a heat exchanger 50, to a mixer 19 in a pipe 20 that feeds a small amount of exhaust from an exhaust pipe 21 to a hydrogen generator 22.
The hydrogen generator 22 may be a catalytic partial oxidizer (CPOx), a non-catalytic (homogeneous) partial oxidizer, or an auto thermal reformer (ATR). Within the hydrogen generator, if it is a CPOx, foam monolith or other form of catalyst, which may comprise a group VIII metal, preferably nickel, cobalt, rhodium, iridium or platinum, convert fuel along with hydrocarbons, water and oxygen into a mix of hydrogen, CO and CO2, which is regeneration gas, commonly called xe2x80x9csyngasxe2x80x9d. This is provided through a conduit 26 to a pair of NOx adsorbent traps 35, 36 which are alternatively connected by corresponding valves 40-43 to either the conduit 26 with hydrogen-containing gas from the generator 22, or to a pipe 48 containing engine exhaust.
The valves are controlled so that engine exhaust is allowed to flow in one of the traps 35, 36 for a period of time which is less than the time necessary to saturate it with NOx, and then the valves are switched so that exhaust flows in the other NOx trap, while the first NOx trap is regenerated by the hydrogen and carbon monoxide from the generator 22. In one regeneration cycle, the valves 41 and 42 will be closed and the valves 40, 43 will be open so that engine exhaust is adsorbed in the trap 35, and the trap 36 is regenerated; in the next regeneration cycle, valves 40 and 43 will be closed and the valves 41 and 42 will be open so that engine exhaust is adsorbed in the trap 36, and the trap 35 is regenerated, and so forth.
Although various adsorbents may be used, the NOx traps may, for example, contain barium carbonate (BaCO3) as the adsorbent. Typically, a catalyst, such as platinum, may be wash-coated on the adsorbent material to catalyze the reaction. When the diesel exhaust is adsorbed by the barium carbonate, a reaction generates barium nitrate.
2NOx+BaCO3xe2x86x92Ba(NO3)2+CO2
Then, during the regeneration cycle, the barium nitrate is converted back to barium carbonate, as follows:
3H2+2CO+Ba(NO3)2xe2x86x92BaCO3+N2+3H2O+CO2
The heat exchanger 50 causes heat of the engine exhaust to vaporize the fuel in the line 18 before applying it to the hydrogen generator, which is particularly useful in the case of a CPOx reformer being used as the hydrogen generator.
A CPOx reformer is preferred in one sense because it is very small and can run with low steam carbon ratios and high turndown ratios without soot or carbon formation. However, diesel engine exhaust contains particulates (soot) and oxides of sulfur (SOx), which may deactivate the CPOx catalyst over a period of time. Therefore, a non-catalytic (homogeneous) partial oxidizer may alternatively be selected as the hydrogen generator 22. The percentage of hydrogen produced is only slightly less than that produced by a CPOx. It is easily started by employing a simple spark plug, as is known. Additionally, POX is cheaper than CPOx; control of the O2/C ratio is known (similar to engine O2/fuel ratio), and simpler; SOx and soot do not affect it; and there is no steam/C ratio problem.
However, the alternating adsorption and regeneration cycles require large, high temperature valves for the engine exhaust. Switching of the exhaust from one adsorption bed to the other, at high exhaust temperature, is a difficult operation.
Furthermore, the engine exhaust valves leak: typically on the order of 5% of the total engine exhaust will flow through the wrong adsorption bed during regeneration thereof. Because there may be up to 15% oxygen in the engine exhaust, which oxygen will react with the hydrogen and carbon monoxide in the regeneration gas, a significant amount of regeneration gas is consumed (wasted) by being combined with oxygen due to the leaks in the valves. The reaction of O2 with H2 and CO will cause a rise in temperature which could deactivate the NOx adsorption bed catalyst.
Objects of the invention include: eliminating high temperature valves in regenerating adsorption beds; improvement in the reduction of NOx to nitrogen and other harmless gases in internal combustion engine exhaust; providing a continuous process for regenerating NOx adsorbents; reducing the size and complexity of NOx-reducing equipment for engine exhaust; simplified equipment for meeting EPA 2007 NOx emission requirements; and avoiding waste of regeneration gas that occurs due to valve leakage in alternating NOx-adsorption systems.
This invention is predicated in part on the discovery that the amount of time that it takes to regenerate an NOx adsorption bed when exposed to regeneration gas is much less than the amount of time that the same size of adsorption bed may take to become saturated with NOx, when in the flow of engine exhaust.
According to the present invention, a relatively rotating inlet gas distributor and NOx adsorption bed having a plurality of flow paths lined with adsorption catalyst, causes a flow of internal combustion engine exhaust in each path during a first fraction of a revolution, and a flow of regeneration gas in each path during the remainder of a revolution. The exhaust gas and regeneration gas are both flowed continuously through the bed.
According to the invention, the flow of gases into the bed is controlled by a distributor having a baffle therein to keep the exhaust gas and regeneration gas separate, and to determine which paths receive one or the other of the gas flows at any point in time. Either the bed (in one embodiment) or the distributor (in another embodiment) may be rotated to cause the gas flows to alternate in the flow paths.
According to the invention in one form, the fraction of each revolution of the bed or distributor devoted to adsorption of NOx is much greater (on the order of between two and forty to one) than the fraction of each revolution required for regenerating of the adsorbent.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.